KVN(Koeran VLBI Network) is the first VLBI system in Korea and also the first dedicated millimeter wave VLBI in the world. KVN consists of three stations with 21m radio telescopes, such as Seoul (Yonsei Univ.), Ulsan(Ulsan Univ.) and Jeju(Tamna Univ.).<br /><br />
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In millimeter and sub-millimeter wave observation for VLBI, it is very crucial to correctly calibrate the phase variations of the electromagnetic waves propagating through the troposphere. To overcome this issue, KVN employs unique the multi- frequency bands receiver system which is able to perform the simultaneous observation up to four frequency bands, 22GHz, 43GHz, 86GHz and 129GHz.<br /><br />
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The phase calibration by using multi-frequency bands receiver system is based on that the phase fluctuations from a given amount of water vapor increase linearly with frequency, that is, the troposphere is non-dispersive in terms of phase fluctuations.<br /><br />
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The multi-frequency bands receiver system and the current status of KVN will be described.